It is known that, for this type of shrouded propeller, the performances are characterized by the "lifting efficiency", also called "figure of merit", which corresponds to stationary operational conditions and which is the ratio between the minimum power in order to obtain a given pull (or thrust) and the real power measured. The expression of this figure of merit is given by the following known formula: ##EQU1## in which
FM is the figure of merit,
T the desired pull (or thrust),
P the necessary power to be supplied to the propeller,
.rho. the density of the air,
R the radius of the propeller, and
.sigma. the coefficient of diffusion of the aerodynamic surface flux, this coefficient .sigma. being equal to the ratio ##EQU2## with S .infin. representing the surface of the flux at downstream infinite and S being the surface of the disc formed by the propeller in rotation.
In order to increase the figure of merit at fixed power and dimensions, it is therefore necessary to increase the pull of the propeller. Now, such pull is expressed, for a unit width section, by the following expression: EQU T=1/2.rho.Cz1V.sup.2
in which
.rho. is the density of the air,
Cz the coefficient of lift of the propeller blade section in question,
1 the chord of this section, and
V the speed of rotation of the section of the propeller blade with the radius in question.
In order to obtain considerable pulls with small chords, therefore small propeller masses, the sections are therefore made to operate at high speeds and high coefficients of lift.
Furthermore, for an optimalization of the complete blade, it is generally advantageous, from the standpoint of yield, to have, especially in the case of a shrouded propeller, a spanwise distribution of lift, which increases from the hub up to the end of the blade. The end sections, for which the relative speed is highest, therefore also operate with the highest coefficients of lift of adaptation. It is known that the coefficient of lift of adaptation is the coefficient of lift at which the section must work with a minimum coefficient of drag and for which it is defined.
Furthermore, it is known that, for the known sections, the increase in the speed and the coefficient of lift is translated by an increase in the coefficient of drag and this increase is all the more rapid as the Reynolds number is lower, which is the case for the applications envisaged by the present invention.
The use of known sections therefore leads to considerable losses and the yield of a shrouded propeller presenting such a known section is very low.
Moreover, particularly in the case of such a shrouded propeller being intended for controlling the attitude of a helicopter in manoeuvring flight, it must therefore be capable of furnishing as great a thrust as possible in positive pitch and at a certain level in negative pitch, which means that the blade profiles present high maximum and minimum lift coefficients and the range of coefficient of lift around the coefficient of lift of adaptation, for which the coefficient of drag remains low, must be as extensive as possible.
Now, the conventional sections used for shrouded propeller blades, such as NACA 63 or NACA 16 sections or even more recent sections, do not present good performances concerning the coefficients of maximum and minimum lift and the operational ranges around the coefficient of lift of adaptation are very small.
The object of the present invention is therefore a new family of sections for shrouded propeller blade, allowing the complete definition of the blade and giving the propeller very good performances in the various conditions of use, these performances being the following:
coefficient of lift of adaptation varying from 0 to 1 between the root and the end of the blade;
operational Mach number varying between 0.3 and 0.7 between the root and the end;
range of coefficient of lift for which the coefficient of drag remains of small extent around the coefficient of lift of adaptation;
high maximum and minimum coefficients of lift, these performances being obtained in a range of Reynolds numbers varying from 0.5.times.10.sup.6 at the root and 1.3.times.10.sup.6 at the end.